The invention relates in general to direct tension indicating washers and in particular to direct tension indicating washers that indicate when the proper bolt tension has been achieved.
Standard DTIs, when compressed to the point where a feeler gage will not enter the residual gap between the top surface of the DTI and the underside of the bolt head or compressing flat washer, will show the installers that they are at a satisfactory point in the bolt's “tension/elongation” curve. However, the determination of adequate DTI bump compression by means of a feeler gage is labor intensive and open to inspection judgment as to whether the feeler gage has been refused or not, to hardware quality variances (such as flat washer hardness), to DTI installation errors (placing the DTI upside down with the bumps against the steelwork instead of against a hardened surface like a flat washer or bolt head), and to problems of hole size variability (where too large a hole allows the standard DTI to collapse into the hole, giving the inspector the impression of feeler gage refusal whereas the “gap” has been closed by means of material, etc.). In addition, the appearance of a compressed DTI is much the same as the appearance of an uncompressed DTI, at least from a modest distance away, and therefore it is imperative that a feeler gage be used to inspect all or virtually all of the DTIs or some bolts will have been missed.
An improvement to the standard non-squirting feeler gage type of DTI is the SQUIRTER DTI, wherein a known quantity of bright orange elastomeric material is deposited under the DTI protrusion, cured, and when the DTI protrusion compresses parallel to the development of bolt tension, eventually the highly visible elastomeric material finds its way out to the outer diameter of the DTI through a small stamped channel in the underside of the DTI. The volume and appearance of the squirted elastomeric material can be related to actual protrusion compression, which is known to be parallel to the development of bolt tension. An exemplary SQUIRTER DTI is disclosed in U.S. Pat. No. 5,931,618, the entire contents of which are incorporated herein by reference.
SQUIRTER DTIs have achieved a strong place in the bolt tensioning marketplace because of their capacity to show relatively unskilled workers that the bolt tension is right, regardless of the torque resistance of the bolt assembly, primarily due to the visible nature of the elastomeric emission. By judging by (first) calibrating the amount of squirted silicone in a known tension bolt calibrator, and then (second) by imitating the amount and appearance of squirted silicone ejected in the calibrating exercise on the bolts being installed in the actual steelwork, the distribution of actual bolt tensions achieved are, to a high confidence level, (a) above the minimum required, and (b) not so much higher than the minimum as to approach the “caution” level which is close to the point where the bolt reaches maximum capacity and then begins to loose capacity. The SQUIRTER DTI's visual emission of elastomeric material, when correctly used as a measure of protrusion compression, is intuitive, is independent of language interpretation and therefore useable in many countries, and also allows bolt installers to see that they have correctly installed all of the bolts, and not missed some.
There are situations where it is desirable to know that bolt tension has surpassed a first level and reached some second level. U.S. Pat. No. 6,425,718, hereby incorporated by reference herein, describes a DTI having bumps (or protrusions) of multiple heights to indicate different bolt tensions. While well suited for its intended purpose, improvements may be made to the design in U.S. Pat. No. 6,425,718.